Radio communication apparatuses used in radio base stations include power amplifiers in order to amplify the powers of transmission signals that are wirelessly transmitted. Since the power amplifiers are generally requested to perform high-output operations for radio-frequency input signals, the power amplifiers are desirable to have high power amplification efficiency for the radio-frequency input signals. The power amplifiers having the high amplification efficiency are realized by, for example, Class F power amplifiers.
FIG. 1 illustrates an example of the configuration of a power amplifier in related art. Referring to FIG. 1, a power amplifier 100 includes a transistor 102, an input matching circuit 104, an output matching circuit 106, a gate bias generating circuit 108, and a drain bias generating circuit 110.
A certain gate bias voltage is supplied from the gate bias generating circuit 108 to the gate of the transistor 102 through the input matching circuit 104, and a certain drain bias voltage is supplied from the drain bias generating circuit 110 to the drain of the transistor 102 through the output matching circuit 106. The transistor 102 receives an input signal supplied to an input node Pin of the power amplifier 100 at the gate thereof and amplifies the power of the received input signal. The transistor 102 supplies the signal resulting from the power amplification to an output node Pout of the power amplifier 100 from the drain thereof as an output signal.
The output matching circuit 106 is coupled to the drain of the transistor 102. The output matching circuit 106 functions as a harmonic processing circuit that processes a harmonic included in the output signal from the transistor 102. When the power amplifier 100 operates as a Class F amplifier, harmonic processing is performed so that the impedance when a load circuit side (the output matching circuit 106 side) of the transistor 102 is viewed from an output node thereof is equal to zero (is short-circuited) for even-order harmonics and is equal to infinity (is opened) for odd-order harmonics in the output matching circuit 106. When the power amplifier 100 operates as the Class F amplifier, such harmonic processing allows the overlap between the voltage waveform and the current waveform of the output signal to be reduced. As a result, it is possible to reduce the power loss to improve the efficiency of the power amplification. Methods of realizing the harmonic processing circuit by a distributed constant line composed of multiple stubs are disclosed in, for example, Japanese Laid-open Patent Publication No. 2001-356779 and Japanese Laid-open Patent Publication No. 2003-234626.
The input matching circuit 104 is coupled to the gate of the transistor 102. The input matching circuit 104 functions as a harmonic processing circuit that processes a harmonic included in the input signal into the transistor 102. Harmonic processing similar to the one performed in the output matching circuit 106 is performed also in the input matching circuit 104 in the power amplifier 100. In the power amplifier 100, performing the harmonic processing not only in the output matching circuit 106 at the output side but also in the input matching circuit 104 at the input side allows the efficiency of the power amplification to be further improved.
In addition, inverse Class F power amplifiers are also proposed, in which the harmonic processing is performed so that the impedance when the load circuit of the transistor 102 is viewed from the output node thereof is equal to infinity (is opened) for the even-order harmonics and is equal to zero (is short-circuited) for the odd-order harmonics in the in the output matching circuit 106 to reverse the relationship between the voltage waveform and the current waveform of the output signal with respect to the one in the Class F amplifiers in order to improve the efficiency of the power amplification. For example, “Analysis of class-F and inverse class-F amplifiers” (A. Inoue, et al, IEEE MTT-S Int. Microwave Symp. Dig., Boston, Mass. June 2000, pp. 775-778) is known as a related art concerning this proposition. It is considered that the reverse Class F power amplifiers are capable of achieving more efficient power amplification, compared with the Class F amplifiers.